FIG. 1 is a diagram showing a configuration of a conventional electric-discharge lamp lighting device. This electric-discharge lamp lighting device includes a dc power supply 1 for outputting a dc voltage VDC by receiving a electric power from an ac power supply Vin, an inverter 2 for outputting a high-frequency voltage Vcoil by receiving a electric power from the dc power supply 1, a voltage controller 13 for controlling variability of the output voltage Vcoil from the inverter 2 by controlling a switching frequency of the inverter 2, a load 3 including an induction coil 5 connected to an output of the inverter 2 and an electrodeless discharge lamp 6 provided adjacent to the induction coil 5, and a current controller 17 for controlling the switching frequency so as to maintain an output power of the electrodeless discharge lamp 6 at an approximately constant target value.
The dc power supply 1 includes a rectifier diode bridge DB and a boost chopper circuit including a switching element Q6, an inductor L10, a diode D10, a controller 10, and a smoothing capacitor C10. The inverter 2 includes switching elements Q3 and Q4, and an inductor Ls and capacitors Cp and Cs that are elements of a resonant circuit. The electrodeless discharge lamp 6 includes a clear spherical glass bulb or a spherical glass bulb of which an inner surface is coated with phosphor, filled with discharge gas such as inactive gases and metal vapor (for instance, mercury and rare gases). The induction coil 5 is provided adjacent to the electrodeless discharge lamp 6. The inverter 2 applies a high-frequency current of several tens of kHz to several MHz to the induction coil 5, whereby a high-frequency electromagnetic field is produced by the induction coil 5 and a high-frequency power is supplied to the electrodeless discharge lamp 6. According to this, a high-frequency plasma current is produced in the electrodeless discharge lamp 6 so as to produce ultraviolet or visible light.
As shown in FIG. 2, a drive circuit 11 includes a constant voltage source Es, a voltage control oscillator VCO, and resistors R10 and R11. An input terminal VI of the voltage control oscillator VCO is supplied with an output voltage of the constant voltage source Es divided by the resistors R10 and R11, whereby an oscillation frequency is altered according to a sink current Io from the respective dividing points. The input terminal VI of the voltage control oscillator VCO is supplied with a voltage depending on the sink current Io, and the voltage control oscillator VCO outputs an approximate square wave drive signal, at a switching frequency finv corresponding to the voltage depending on the sink current Io, with respect to each switching element Q3 and Q4 mutually shifted in phase by approximately 180° between an Hout terminal and an H-GND terminal and between an Lout terminal and an L-GND terminal.
The voltage controller 13 includes an integrator including an operational amplifier Q1, a resistor R1, and a capacitor C1, a switch SW0 for discharging a charge of the capacitor C1, and the like.
The current controller 17 includes an integrator including an operational amplifier Q9, a resistor R10, and a capacitor C11, resistors R5 and R6 for producing a reference voltage, and the like. The current controller 17 differentially amplifies a signal from a resistor Rd for detecting a resonance current of the inverter 2.
The drive circuit 11 varies the switching frequency according to a sum (=Io) of sink currents Isw, Ifb and Ivr flowing into the voltage controller 13, the current controller 17 and a variable resistor VR from the input terminal VI.
Variable resistor VR is performed to absorb deviations in circuit components, such as the resonant circuit of the inverter 2 and load 3, and the drive circuit 11. When a frequency sweep control (described later) is performed by the voltage controller 13, the variable resistor VR is adjusted for setting an appropriate frequency variation range thereby performing a stable starting and lighting. The following are descriptions of operations with reference to FIGS. 4 and 7. Note that, in FIGS. 4 and 7, a reference sign “a” (solid line) represents a case with a variation of a load impedance, and a reference sign “b” (dashed line) represents a case with no variation of the load impedance. When the switch SW0 is switched from an ON-state to an OFF-state, the voltage controller 13 charges the capacitor C1 by supplying a electric power from a dc voltage E1 via the resistor R1, applies a voltage VC1 at both ends of the capacitor C1 to a non-inverting input terminal of the operational amplifier Q1, and varies the voltage VI according to the voltage VC1 at both ends of the capacitor C1, so as to perform the frequency sweep control (start frequency fs→end frequency fe) of the drive circuit 11.
When it is assumed here that the relationship between the input voltage VI and switching frequency finv of the drive circuit 11 is set to have an inclination shown in FIG. 3, the voltage VI is increased since the sink current Io (=Isw) is decreased when the voltage VC1 is increased. As a result, the switching frequency finv is gradually decreased. Therefore, the voltage Vcoil is gradually increased when the switching frequency finv and the voltage Vcoil have a relationship shown in FIG. 7. In addition, since the electrodeless discharge lamp 6 is configured to exceed a voltage minimally necessary to start ignition during the frequency sweep, the electrodeless discharge lamp 6 is lighted at a certain switching frequency finv (=fi), and the voltage Vcoil is decreased immediately, thereby shifting to the lighting side on the curve line in the figure.
Note that, during the frequency sweep control, the resonance current of the inverter 2 is increased from an initial state (≅zero). Thus, an output of the operational amplifier Q9 is decreased from the high voltage at the initial state. However, the operational amplifier Q9 produces a delay time to an operation of a differential amplifier due to the function of the resistor R10 and the capacitor C11 of the integrator. Therefore, while the output Vout of the operational amplifier Q9 is high compared with the voltage VI of the input terminal of the drive circuit 11, the sink current Ifb from the drive circuit 11 becomes approximately zero and a mask operation is performed, thereby resulting in the current Io≅Isw+Ivr. Accordingly, the switching frequency of the drive circuit 11 is controlled by the output of the voltage controller 13.
Then, after the electrodeless discharge lamp 6 is lighted at the time t=t2, the switching frequency keeps varying until the voltage VC1 becomes a constant value. In this case, after the electrodeless discharge lamp 6 is lighted when the switching frequency is “fi” in FIG. 7, the switching frequency keeps varying until the end point of the frequency, “fe”. However, the operational amplifier Q9 performs a negative feedback operation thereby the current Ifb flows out at the time t>t3 due to the decrease of the output Vout of the operational amplifier Q9. Then the switching frequency finv is increased, and the frequency is controlled until resulting in a predetermined resonance current of the inverter 2 arranged according to a reference voltage determined by the resistors R5 and R6, thereby resulting in a certain frequency fx. Due to such a feedback control, it is possible to maintain the output power of the inverter 2 at an approximately constant predetermined value.
In addition, as shown in FIG. 12, the load 3 may be other electric-discharge lamps such as a construction including an electric-discharge lamp (fluorescent light) FL having filaments F1 and F2 and a capacitor C20, and the like. Such electric-discharge lamps perform the similar operations.
In particular, the load of the electrodeless discharge lamp is an inductor load when starting ignition. Thus, a larger voltage and power is required when starting compared with other light sources such as a fluorescent light with electrodes. Accordingly, it is necessary to set a Q factor of the resonant circuit of the inverter 2 to be high in order to start and light stably. However, when there are some factors for a load impedance variation of the inverter 2 such as a change in ambient temperature and an approach of a metal housing toward a circumference of the electrodeless discharge lamp 6, the voltage Vcoil is greatly altered, which results in difficulty in starting and lighting stably. Therefore, by starting by the frequency sweep, it is possible to start and light stably since an influence of the load impedance variation can be absorbed to some extent. Consequently, starting by the frequency sweep can be effective especially when the electrodeless discharge lamp is employed.
Japanese Patent Laid-Open Publication No. 2005-063862 discloses a start-up of a load of an electrodeless discharge lamp using frequency sweep, and discloses a configuration to improve start-up performance by applying a sufficient voltage even if constants of components are altered by sweeping from a higher frequency than an actual resonance frequency toward the resonance frequency.
However, when the inverter 2 and the load 3 include the resonant circuit, and when the electric-discharge lamp is started and lighted being supplied with power by the switching frequency control by use of the resonance property, a resonance output of the resonant circuit included in the inverter 2 and the load 3 varies as an impedances of the inverter 2 and the load 3 vary. Thus, there was a problem that a stable starting and lighting could not be achieved because of an insufficient power supply to the electric-discharge lamp, an occurrence of dying out, and the like. As for factors for such impedance variation, a change in ambient temperature, a variability and time-dependent change of constants of circuit components, an approach of a metal housing toward the load 3, and the like are included.
Specifically, when the electrodeless discharge lamp 6 is uses as the load 3, the impedance variation is significantly occurred because of the approach of the metal housing. FIG. 6 shows an example of a downlight using the electrodeless discharge lamp 6. The electrodeless discharge lamp 6 in the downlight is covered with a reflecting plate 30. When the reflecting plate 30 has high conductivity especially being made of metal, an induced current 31 by electromagnetic induction from the induction coil 5 circularly flows on the reflecting plate 30. Thus, an inductive component by the reflecting plate 30 is produced, which results in a parallel connection of the inductor to the induction coil 5 in an equivalent circuit. Consequently, the resonance curve lines when starting and lighting as shown in FIG. 7 are shifted to the high frequency side compared with a case not including the metal housing such as a downlight.
In the case with the variation of the load impedance in FIG. 7 (shown as the solid line, the reference sign “a”), the frequency varies until the capacitor C1 is fully charged and the voltage VC1 becomes the constant value even after the electrodeless discharge lamp 6 is lighted. Therefore, the frequency varies until the end point frequency fe. However, since the resonance curve line is shifted to the high frequency side, the frequency sweep is performed until over a peak of the resonance curve line when lighting and further until an area where the output voltage Vcoil is decreased. Thus, the end point frequency fe results in a lower frequency than the peak of the resonance curve line when lighting. In such an area, the switching frequency control by a negative feedback operation using the operational amplifier Q9 of the current controller 17 is to be unable to function because of an increase and decrease relationship between the switching frequency and the output power (or the resonance current of the inverter 2). As a result, there were problems such as that the output power could not become a predetermined value, and the electrodeless discharge lamp 6 was dying out.
As a method to avoid such a matter, it may be applicable that the end point frequency fe is adjusted while the electrodeless discharge lamp 6 is fixed to an apparatus surrounded by the metal housing. However, it is necessary to perform such adjustment for each case in order to adapt to a variety of apparatuses. This is not substantive because of much time and high cost.
The present invention has been made to solve the above-mentioned problem. It is an object of the present invention to provide an electric-discharge lamp lighting device and a lighting fixture thereof capable of starting and lighting stably even when a load impedance of the discharge lamp varies.